In motor vehicle/trailer combinations, depending on the travel velocity, crosswind conditions, tires, and load, the trailer has a more or less pronounced tendency to dynamic instability which is referred to as “trailer swings” and expresses itself in that the trailer swings to the right and left around the extension of the central axis of the towing vehicle.
A driver assistance system is described in German Patent Application No. DE 199 01 953, in which the instability of the trailer is measured continuously and, if it exceeds a permissible value, a braking operation is triggered automatically to bring the combination below a critical velocity, at which the swinging oscillations die out again.
The sensor unit for detecting the instability is formed by two ultrasonic sensors which are installed in different transverse positions in the rear of the towing vehicle and measure the distance to the trailer. By comparing the distance signals obtained by the various ultrasonic sensors, the current diagonal position of the front of the trailer in relation to the towing vehicle may be detected, so that oscillations of the trailer may be identified.
Electronic driver assistance systems are also known for motor vehicles, via which it is possible, for example, to measure the distance to a preceding vehicle with the aid of a radar sensor and to regulate the velocity of the host vehicle on this basis in such a manner that the preceding vehicle is followed at an appropriately safe distance (ACC; adaptive cruise control). Other driver assistance systems are used to recognize an imminent collision with another vehicle and then automatically trigger emergency braking so that the collision may be avoided or at least its consequences may be reduced. These types of driver assistance systems also have an assistance function, which automatically triggers and controls a braking operation under specific conditions.